If people are finding out about these places using Yelp, Google, Groupon et al. is there really a need for a SEO friendly website? Many of these businesses don't care about a web presence because its effect would be negligible.
I suppose you could justify the cost of building out a website for the local dry cleaner, but seriously, does that help it gain any new business? I doubt it.
It probably would. Remember that we're a very mobile society (in the US) and towns have new arrivals every day.
If I just moved to a new town, I probably don't know where the nearest dry cleaner, or Thai restaurant, or carpet cleaner is. Rather than spend valuable time driving around searching, my first inclination will be to google it. If they get my business and do a good enough job, odds are I'll continue to patronize them for years. People rarely change service providers unless they move or there's a major problem.
If you Google it, you'll get a comprehensive list of dry cleaners/Thai restaurants with an accompanying map. And that list usually shows up at the very top rendering everything below meaningless. Try it.
I suppose you could justify the cost of building out a website for the local dry cleaner, but seriously, does that help it gain any new business? I doubt it.